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Phosphate Removal – the Development of Feed Forward

Chemical Dosing Control

Andrew WallaceProduct Specialist Analysers

What are Phosphates, where do they come from?

• Phosphates (PO4) are the natural form of elemental Phosphorus (P) and are classified as orthophosphates (pyro, meta- polyphosphates), and organically bound phosphates.

• They occur in solution, in particles or detritus, or the bodies of aquatic organisms.

• Both organic and inorganic phosphorus can be dissolved in the water or exist as a suspension (attached to particles in the water column).

• They can come from potable water treatment, laundering, boiler water treatment, fertilizers and discharges from biological processes – sewage

treatment works etc.

The Phosphorus Cycle

Definition of Phosphorus and Phosphates

• Orthophosphates• Applied to agricultural or residential cultivated land as fertilizers are

the most soluble form and are found in storm water and surface water.

• Condensed phosphates• Added to water supply during treatment, larger amounts used in

laundries and other cleaning systems. Treatment of boiler water in power industry.

• Organic bound phosphates• Formed primarily by biological processes but can be formed from

orthophosphates in sewage treatment.

Terms used in the Analysis of Phosphorus

• Soluble Phosphate:– Filtration through a 0.45-micron membrane filter separates dissolved from

suspended forms of phosphorus – this is not absolute but has been taken as a convenient replicable analytical technique for this determination.

– Phosphates that respond to colorimetric tests without preliminary hydrolysis or oxidative digestion of the sample are termed “REACTIVE PHOSPHORUS”.

– While reactive phosphorus is largely a measure of orthophosphate a small fraction of any condensed phosphate present is usually hydrolysed in the procedure. Reactive phosphorus occurs in both dissolved and suspended forms.

Terms used in the Analysis of Phosphorus

• Condensed phosphates:– Acid hydrolysis at boiling water temperature converts dissolved and

particulate condensed phosphates to dissolved orthophosphate. The hydrolysis unavoidably releases some phosphate from the organic compounds this can be minimised by judicious selection of the acid strength, hydrolysis time and temperature.

– The term “ACID HYDROLYZABLE PHOSPHORUS” is the preferred term for this fraction

• Organically bound phosphates:– Phosphate fractions that are converted to orthophosphate only by oxidation

destruction of the organic matter present are considered “ORGANIC” or “ORGANICALLY BOUND” phosphorus.

– The severity of the oxidation required for this conversion depends on the form and to some extent the amount of the organic phosphorus present. It occurs in both dissolved and suspended forms.

Development of the System

• 2004 Trials unit at Thames Water – basic system design• 2005 Pre-production design trials unit at Wessex Water• 2006 First production system installed in Wessex Water• 2006 Sampling system updated• 2006 Production Systems sold to Anglian Water, Astra Zeneca, Yorkshire

Water• 2007 Systems sold to South West Water, Wessex Water, Southern Water• 2008 Development of PLC Control and Electrical panels• 2008/9 Systems sold to Wessex Water, Yorkshire Water, Anglian Water,

South West Water and Northern Ireland Water• 2010 Systems sold to Wessex Water and Anglian Water

The Original Trial Site

9

Initial Trials Units

First Development system

Redesign for Trials unit

The Original Sample Pump!

The Critical Part of the System

Dosing Points

Final Effluent Ortho-Phosphate

Inlet Profile – Original Site

Inlet Daily Profile – Original Site

Inlet Turbidity Vs Ortho-Phosphate

0

200

400

600

800

1000

1200

5/4/06

13:10

5/4/06

16:30

5/4/06

19:50

5/4/06

23:10

5/5/06

2:30

5/5/06

5:50

5/5/06

9:10

5/5/06

12:30

5/5/06

15:50

5/5/06

19:10

5/5/06

22:30

5/6/06

1:50

5/6/06

5:10

5/6/06

8:30

5/6/06

11:50

5/6/06

15:10

5/6/06

18:30

5/6/06

21:50

5/7/06

1:10

5/7/06

4:30

5/7/06

7:50

5/7/06

11:10

5/7/06

14:30

5/7/06

17:50

5/7/06

21:10

5/8/06

0:30

5/8/06

3:50

5/8/06

7:10

5/8/06

10:30

5/8/06

13:50

5/8/06

17:10

Date & Time

FTU

0.0

2.0

4.0

6.0

8.0

10.0

12.0

mg/

l as

P

Turbidity Phosphate

Is there a relationship with Solids?

Flow ONLY control !!

0

50

100

150

200

250

300

350

400

D a y

Ferri

c us

e (li

tres)

D i u rn a l D o s i n g c o n t r o l

D i re c t c o n t r o l p h a s e 1

25% saving

Di r e c t c o n t ro l p h a s e 2

33 % saving

Cost Saving – Original Site

Inlet Ortho-Phosphate Levels

12th May

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

00:00 01:00 02:00 03:00 04:00 05:00 06:00 07:00 08:00 09:00

Time

mg/

l as

P

-20.000.00

20.0040.00

60.0080.00

100.00120.00

140.00160.00

180.00

l/sec

Conc PPump Flow l/hrFlow l/sec

Data acquired and used for analysis

Dose Compaison 4th to 24th May

0

50

100

150

200

250

300

350

400

D a t e

Cost saving 2nd Trial Site

22

Diurnal Pattern

23

First Production System

Installed on Inlet ChannelPhosphate Monitor and Accessories

24

Results from Keynsham

11th Oct P, Dose, Flow

0.00

2.00

4.00

6.00

8.00

10.00

12.00

00:0000

:5001

:4002

:3003

:2004

:1005

:0005

:5006

:4007

:3008

:2009

:1010

:0010

:5011

:4012

:3013

:2014

:1015

:0015

:5016

:4017

:3018

:2019

:1020

:0020

:5021

:4022

:3023

:20

Time

mg/

l as

P

-50.00

0.00

50.00

100.00

150.00

200.00

250.00

l/sec

; l/h

r (D

ose)

11/10/2005 Dose Flow

12th October Flow, P and Dose

0.001.002.003.004.005.006.007.008.009.00

00:0000

:5001

:4002

:3003

:2004

:1005

:0005

:5006

:4007

:3008

:2009

:1010

:0010

:5011

:4012

:3013

:2014

:1015

:0015

:5016

:4017

:3018

:2019

:1020

:0020

:5021

:4022

:3023

:20

Time

mg/

l as

P

-50.00

0.00

50.00

100.00

150.00

200.00

250.00

l/sec

:l/h

r (D

ose)

12/10/2005 Dose Flow

25

Results from Keynsham

13th All Charts

0.001.002.003.004.005.006.007.00

00:0000

:5001

:4002

:3003

:2004

:1005

:0005

:5006

:4007

:3008

:2009

:1010

:0010

:5011

:4012

:3013

:2014

:1015

:0015

:5016

:4017

:3018

:2019

:1020

:0020

:5021

:4022

:3023

:20

Time

mg/

l as

P

0.0020.0040.0060.0080.00100.00120.00140.00160.00180.00

l/sec

; l/h

r (D

ose)

13/10/2005 Dose Flow

Original dose

26

Wessex Water DataDose Rates From SCOPEX

0

5

10

15

20

25

30

35

Da t e

Dose mg/ l Flows & Tank Levels

RATIO 3 :1 RATIO 2 . 5 :1

CONTROL

Results

• Initial trial results for a 4 week period showed savings of between 30 - 40 % of chemical dosed

• Results since full production system installed (36 months) have showed consistent savings of 20 – 25% in chemical and NO consent failures

• Higher savings shown during high flow periods where dosing profile formally based on flow with capping at a high flow

• The Fe to P ratio changes during the winter and summer seasons in this case 2.8 for winter and 2.4 for summer

• Operator cleans filter element every 3 weeks• Support technician changes reagents every 5 or 10 weeks• Analysis every 30 or 60 minutes

28

Analytical Method

• Ortho-Phosphate is the form of phosphorus measured using the standard “Blue Book” method of Molybdenum Blue and is reported as P not PO4

• The reason for this is two fold:– Fast simple chemistry– Measures just the dissolved Phosphates– Consent is Total Phosphorus and traditionally ortho-phosphate is

measured by test kit on the final effluent, there is a relationship between ortho-phosphate and Total phosphorus which is site related. As a rule of thumb the Total Phosphorus is 0.3 to 0.5 higher than the ortho-phosphate when quoted as P.

• PO4 relationship to P is:– 31 +(16)*4 = 95 (atomic weight)– 95/31 = 3.06– Therefore the result for PO4 is 3.06 times higher than P

29

Why we succeed

• The reason we succeeded in the analysis of crude sewage:

• The sample is filtered and then diluted so this removes all the turbidity effects and allows us to use the simple chemistry as the method is only linear to 6 mg/l as PO4 but our dilution is at least 1 in 10.

• We do not run the filter unit continuously but control the complete cycle from the analyserand it’s associated controller /PLC.

• Note:• The determination of Total Phosphorus in the crude sewage could NOT be performed on line as

much of the phosphorus is bound up in the solids and therefore the solids would have to be sampled and analysed as well. As part of the set up / commissioning of this system an analyser is situated at the final effluent and the Iron to P ration can be adjusted to achieve a compliant final effluent which will take into account total phosphorus and solids.

30

Relationship of Iron and Phosphate

• Iron salts will precipitate phosphate in the form of an iron phosphate complex, in pure chemistry the relationship is 1 : 1 but in sewage treatment it can be as much as 1 : 3 depending on the sewage coming in and the form of the iron being used.

• Iron salts will also act as a flocculating agent for solids therefore on a plant using iron the solids will increase, the level of solids will also affect the efficiency of the iron in reducing phosphate.

31

System Components

• Kiosk• Sampling System – pump etc• Filter Unit, Sample pot• De-ioniser• Controller – Sampling, Dosing • Electrical & Telemetry Panel• Monitor

32

Kiosks non walk in

Inlet System Final Effluent System

33

Kiosk Walk-in

Latest Inlet System Kiosk

35

“Mini” kiosk

Final Effluent System where area is sensitive to structures

Sampling Systems

• Robust submersible pump• Macerator • High flow rate • Pump only runs for 60 to 90 seconds per cycle• 240 or 415 volt versions• ATEX pump available

37

Special Sampling System

• ATEX 415 volt pump • Sample from high pressure

pumped riser• Multiple valve control• Sample present sensors• Flush through cycle

38

Filtration

• Stainless steel “wedge wire”filter element

• Simple design for easy cleaning and servicing

• Central air blast for cleaning during every sample cycle

• Sample pumped to sample pot

39

New Style Compressor

40

Sample Pot & Peristaltic Pump

• Sample pot designed with secondary filter material to protect analyser

• Sample overflow with small sample chamber for flush out of previous sample

• Peristaltic pump used to draw up sample from filter or sample chamber (in final effluent applications)

• Multiple sample points (2 in the case illustrated)

41

Sample Pot Components

Filter Wool Holder

Lid

Sample Pot Body

Overflow

Filter Wool

42

On line De-ioniser

• Where a tap water supply is available an online de-ionisercan be used in place of large dilution water containers

• Special design to produce a small volume of dilution water as required

• Colour indicator resin to indicate condition of de-ionisercartridge

43

Controller

• Two types of controller available• Relay based with fixed program• PLC based with operator variable

programming• Controller is now integrated into

the electrical panel

44

Electrical & Telemetry Panel

Original Controller panel New design of panel

45

Monitor

• Large integral display and keypad

• Separate electrical compartment

• Integral reagent storage• Multiple chemistry modules• Low voltage system

46

Chemistry Module

• Standard “blue book chemistry method”

• Robust chemistry module• Heated reaction• Automatic calibration and Reagent

Blank• Chemical clean of flow cell

automatically on every calibration• Single non flow dependent

peristaltic pump• Visible reaction in perspex

cylinders

47

Feed Forward Cycle of Events

• Take sample• Analyse sample• Read flow rate• Compute flow rate & phosphate level and apply factor• Combined flow phosphate 4-20 mA signal generated and sent to dosing

pump or control system• Chemical dosed to influent• Final effluent phosphate level monitored and used to check the iron to

phosphate ratio

48

Feed Forward ChartDataDose Ratio Required 3.0 kg Fe / kg P Adjustable Function Via HMIFerric Solution Density 1600 No dosing is required, Dosing Pumps to be inhibited.Solution Iron Content 13 % by weight When inlet ortho P concentration is less than or equal to 0.6 mg/l as P. Because, at 63% orthoP in crude, 0.6 mg/l represents an Inlet total P of less than 1.0 mg/l.Iron content per litre 0.21 kg Fe/l Also dosing pump minimum dose rate is assumed to be approximately 40 l/h, without adjusting the stroke length, thus if required dose rate calculated is less than 40 l/hr, pump is inhibited.

Calculation steps l/s x mg P/l = mg P /smg P/s x 3600 = mg P /hrmg/hr / 1e06 = kg P /hkg P /h x 3 = kg Fe /hkg Fe /h / 0.21 = l / hr mistrale dose rate

Inlet Orthophosphate Concentration as mg P /l0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2

SignalInlet Crude Flow rate 4-20 mA 4.00 4.21 4.43 4.64 4.85 5.07 5.28 5.49 5.71 5.92 6.13 6.35 6.56 6.77 6.99 7.20 7.41

l/s Dose Rate of Ferric Solution (l/hr)30000 347 4.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.032000 370 4.40 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.034000 394 4.80 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.036000 417 5.20 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.038000 440 5.60 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.040000 463 6.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.042000 486 6.40 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.044000 509 6.80 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.046000 532 7.20 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.048000 556 7.60 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.050000 579 8.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.052000 602 8.40 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.054000 625 8.80 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.056000 648 9.20 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.058000 671 9.60 0.0 0.0 0.0 7.0 13.9 20.9 27.9 34.9 41.8 48.8 55.8 62.7 69.7 76.7 83.7 90.6 97.660000 694 10.00 0.0 0.0 0.0 7.2 14.4 21.6 28.8 36.1 43.3 50.5 57.7 64.9 72.1 79.3 86.5 93.8 101.062000 718 10.40 0.0 0.0 0.0 7.5 14.9 22.4 29.8 37.3 44.7 52.2 59.6 67.1 74.5 82.0 89.4 96.9 104.364000 741 10.80 0.0 0.0 0.0 7.7 15.4 23.1 30.8 38.5 46.2 53.8 61.5 69.2 76.9 84.6 92.3 100.0 107.766000 764 11.20 0.0 0.0 0.0 7.9 15.9 23.8 31.7 39.7 47.6 55.5 0.0 0.0 0.0 0.0 0.0 0.0 0.068000 787 11.60 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.070000 810 12.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.072000 833 12.40 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.074000 856 12.80 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

kg/m3

m3/d

49

Monitoring Final Effluent

• Portable monitor allows for 24 hour separate sampling with results available on line

• Remote access available through modem link

• Can be configured to monitor phosphate and iron or aluminum whichever is being dosed

• Can be left unattended for 7 days• Can be powered by battery (this

will require more frequent support)

50

Portable

Sample Pump

Sample Pot

Analyser

Modem Aerial

Dilution Water

Reagents

51

Portable in Action

• Final effluent channel• 4 meters below analyser• Minimal installation• Remote access for data and

control• Simple sampling system

52

Results from Portable Analyser

04/08/08 10:0005/08/08 10:00

06/08/08 10:0007/08/08 10:00

08/08/08 10:0009/08/08 10:00

10/08/08 10:0011/08/08 10:00

12/08/08 10:0013/08/08 10:00

14/08/08 10:0015/08/08 10:00

16/08/08 10:0017/08/08 10:00

18/08/08 10:0019/08/08 10:00

20/08/08 10:0021/08/08 10:00

22/08/08 10:0023/08/08 10:00

24/08/08 10:0025/08/08 10:00

0

1

2

3

4

5

6

7

8

53

Systems Installed

• Anglian Water - Cambridge 3; Fornham, Potton 3• South West Water - Camelford 2, Delabole 2, Scarletts Well 2,

Menagwins, Lodge Hill• Southern Water - Siddlesham• Wessex Water - Keynsham, Saltford 3, Warminster 2,

Fordingbridge, Amesbury, Ratfyn 2, Cranbourne, Pewsey, Netheravon, Abbotsbury, Salisbury 2, Dorchester 2, Downton, Fovant, Great Wishford, Hurdcott, Ringwood 2, Tisbury, Upavon, Wool,

• Yorkshire Water - Malton 2• Northern Ireland Water - Tullagharley• Industrial - Astra Zeneca, Glanbia Cheese

• 12 Inlet, 3 Intermediate, 39 Final Effluent (TP or OP)

54

Conclusion

• Feed forward dosing control results in the optimisation of the chemical being dosed

• Determination of phosphate in crude sewage can be used to control dosing of chemical when combined with the flow rate

• Feed forward control of chemical dosing results in direct savings of chemical and reduced sludge formation

• Fe to P ratio can be optimised• Reduces the possibility of failing discharge consent for both

Phosphorus and residual chemical (Fe or Al)